It is a smallish car (say: low price) I would assume that the reversing switch would not go through a computer control unit. Most likely it is a manual trany. There would be a switch on the trany or the gearshift. It is likely that there is a fuse somewhere. Check in the least read and most discarded book in the world: the Owner's Manual. E
I'll use your post as an example against the least-spec'd variant in the range of vehicles I regularly work on...
This is a model year 2007 onwards, 1000cc engine (yep one Litre) 5-speed manual transmission, 3 or 5 door small hatchback.
It is quite cheap.
It is equipped with CAN-Bus.
There is no fuse directly in the reversing light circuit.
There is, however, a switch on the transmission.
Operation is as follows:
The N.O. contact switch on the transmission is wired directly to ground on one terminal.
The other terminal goes directly to the ECM (Engine Control Module) and does not pass "GO".
When reverse gear is selected, a CAN message is passed from the ECM to the BCM (Body Control Module)
The BCM then switches an output IC (High-Side driver) to supply power to the lamp.
Q. Why use CAN-Bus and multiple ECU's instead of wiring everything directly?
A. Many reasons, some listed below;
Far less copper wiring used, resulting in far less weight.
Switches with lower current carrying capacity can be used - less bulk, less weight.
Fault tolerance and error handling/reporting far superior to direct wiring. (Current-sensing resistors to detect and handle short/open circuits and report faults)
Bulb check facility using "cold pulsing" - where low current, insufficient to actually light a bulb, is passed through the circuit intermittently, to check whether the filament is intact.
Lamp switching facility, i.e when one of a pair of tail lamps in a single cluster is used as Stop/Tail (using PWM with a single filament bulb) and that bulb fails, the Stop/Tail function is switched to the other bulb.
Along with many other reasons.